Is there such a thing as a variable current charger?

[jdege]

Consider a boat with batteries you want to charge from shore power.

Different marinas have different current limits. Some provide 20A, some 30A, some 50A, some provide two outlets you can combine.

If you're cruising, rather than day-sailing out of a single marina, what size charger do you use? If you go with a 15A charger you'll be safe, everywhere, but you'll be charging more slowly than you could, when you plug into a shore outlet that could handle more current. If you go with a 24A charger, you'll get faster charge times when plugged into 30A, but you'll be unsafe plugged into 20A.

Do they make chargers that can be current limited? That you can set to charge more slowly, when the shore power connection can't take higher currents?

 

[wholybee]

The current rating of the marina is the current at AC voltage, not the DC voltage the charger outputs. A120V 15A circuit is the same as about 150A at 12 V. Given efficiency losses etc., you would be safe plugging in a 100A 12V charger into a 15A circuit. With 15A being the smallest you will find, and with 100A probably bigger than any charger you will consider buying, you shouldn't have any issues.

 

[OffGridInTheCity]

You could likely find a bench power supply with CC/CV features that is adjustable. Of course this would likely be bulkier and more expensive than 'just a charger'

 

[Bubba1]

The Aims converter charger does that. The only problem is no one has them in stock.

 

[MisterSandals]

If you go with a 24A charger, you'll get faster charge times when plugged into 30A, but you'll be unsafe plugged into 20A.

I'm not sure I would say "you'll be unsafe" but rather you'd more likely trip a breaker on the shore power pedestal if you try to pull more than the rated receptacle.

But, if you have a 24a charger, that's at 12.8vDC.
24a x 12.8vDC = 307wh

307wh / 120vAC = 2.56a

So you're not even close to tripping a breaker in this scenario, even on a 15a receptacle.

 

[Rider]

Different marinas have different current limits. Some provide 20A, some 30A, some 50A, some provide two outlets you can combine.

The rating of those outlets is simply the maximum amount of AC power you can draw before you blow their breaker. It has nothing to do with the rate at which the charger charges your battery, Your battery charger does automatic DC current limiting when it is connected to the battery, and a typical portable battery charger would be fine on the 20A outlet. The other outlets I'm sure have different receptacles and won't accept a standard Edison plug.

 

[MisterSandals]

<Edit to correct units!>
So for completeness, and seeing as you'd like to be compatible with 15a receptacles...

15a x 120vAC = 1800w
1800w / 12.8vDC = 140a

So yea, i wholy-heartedly agree with wholybee:

A120V 15A circuit is the same as about 150A at 12 V. Given efficiency losses etc., you would be safe plugging in a 100A 12V charger into a 15A circuit.

 

[rmaddy]

15a x 120vAC = 1800wh
1800wh / 12.8vDC = 140a

That should be:

15A x 120VAC = 1800W
1800W / 12.8VDC = 140A

There are no hours in these calculations. Same with your earlier post. Amps x volts = watts, not watt hours.

And the A, V, and W units should be capitalized to be correct (sorry to be that guy).

 

[MisterSandals]

There are no hours in these calculations. Same with your earlier post. Amps x volts = watts, not watt hours.

And the A, V, and W units should be capitalized to be correct (sorry to be that guy).

Thanks for checking up on me! I show the math with hopes folks spot any errors.

Is it really a thing to capitalize A, V and W? I will aspire to do better, thanks for the prod.

My "thing" is folks that can't get LiFePO4 correct... (i could go on!)

 

[rmaddy]

Is it really a thing to capitalize A, V and W? I will aspire to do better, thanks for the prod.

All units have a proper case. While in this forum it's obvious, for example, what 12v means even though it should be written as 12V.

The A, V, and W are capitalized units. They actually come from peoples' names. André-Marie Ampère, Alessandro Volta, and James Watt.
Other units are lowercase such as k (kilo) and h (hour).

Getting the case wrong in some cases is a big deal. m vs M for example (milli vs mega). You wouldn't want the wrong m or M when applied to volts, amps, or watts. Ouch.

Sorry to the OP for getting off-topic.

 

[Q-Dog]

As stated, AC input amps and DC charging amps are not the same thing. Where the 15, 20, 30 or 50 amp AC limits really come into play is when you are trying to run AC loads on the boat from the shore power. It becomes even more critical if you are trying to charge batteries and run loads at the same time. Victron Quattro and Multiplus inverter/chargers, for example, have the ability to limit input amps to whatever amount you choose so as to avoid overloading the shore AC connection.

 

[time2roll]

Half the posts here seem to have a Multiplus which has an input current limit. This is one of the greatest features and seems to be hardly used.

I recommend to set at 80% of the available supply.

 

[Q-Dog]

My Multi is mounted in my house but I vary the input current limit based on whether I am charging from grid or generator. It works great. The power boost function is pretty nice too.

 

[Bubba1]

The Aims converter charger does that. The only problem is no one has them in stock.

 

[Steve_S]

My Samlex EVO 4024 Inverter/Charger pulls 120V/22A from the genset to charge a 24V battery bank at 80A.
The conversion from VAC through a charger to DC is significant.

You did not mention your Battery Type:
Lead FLA/AGM requires 3 stage charging.
LFP (LiFePO4) requires 2 stage charging CC (Constant Current) to get the battery to 90% then CV (Constant Voltage) with decreasing current to top off to designated 100%.
NOTE End-Amps / Tail-Current for LFP is 0.05 of the AH rating. So 100AH=5A, 280AH=14A

 

[Roberto2]

If you are electronically inclined, here is a circuit you could contemplate building;

or this one;

 

[fabio de sal]

One Question. I have seen people here doing the numbers like:
15A x 120VAC = 1800W
1800W / 12.8VDC = 140A

but why 12.8VDC? Don't most LiFePO4 Battery Charger use 14.6V ?
Im looking at buying this one: LiTime 12V (14.6V) 80A Mountable LiFePO4 Battery Charger

Im in Europe and often campsites have 6A limit. In EU we have 220V AC

Would this be correct:
80A × 14.6V = 1168W

Assuming the charger is 90% efficient, the actual power draw from the AC side will be higher.
1168W / 0.9 = 1298W

Now, let’s calculate the AC current draw: 1298W / 220V = 5.9A

Thats very close to the 6A..
Should I still do it or get a smaller batt. charger?

 

[OffGridInTheCity]

You're on the right track but keep in mind that a battery doesn't just have 1 voltage - it's a range. The charger will raise the voltage higher than rest voltage of the battery so that charging current will flow into the battery during the CC (constant current) portion of charging until the battery nears full charge and then the charger will keep a fixed, max voltage and the charging current will gradually slow down to 0a as the battery is full CV (constant voltage) portion of charging.

At lower voltages the the charger will use less power than at higher voltages during the CC portion of the charge. The key is not 12.8v - e.g. that's just a middle of the road 'guide' for approximate power estimates - but the actual max CC voltage * amp for you're situation.... maybe 14.5v @ x amps.... just before it enters CV mode.

And while 90% efficiency is as good as I've seen, not all chargers achieve this. I had a cheap one that was only 65% efficient (they lied) and took way more power.

I have a similar situation in my vehicle - it has 2 x 120v plugs and advertises as 400w so I bought a charger that would pull 380w but found out that each plug can only do 200w and it took me 2 tries to get a charger that would stay just under 200w for the entire charge cycle.

My advice would be to either leave a good buffer (smaller charger as you suggest at the end of you're post) or be prepared to do some experiments to confirm that you're just under the 6a maximum.

A thought - in my case I did 2 x chargers at 180w each - 1 for each plug with parallel charging -> battery. Can you do this as well? do multiple chargers from different plugs where each one has that buffer, maybe 5a each or 3a each and achieve the original 6a goal?

 

[HarryN]

I just use a couple of chargers in parallel.

 

[Warpspeed]

Somewhere in any battery charger there will be a current shunt to measure the current going into the battery.

A charger can be "fooled" into limiting the charging current to something less than its maximum normal full rating by just changing the current shunt.

So buy the biggest charger that is practical to use wherever the highest available shore charging power is available, then have some means of changing the current shunt to something with a lower amp rating at the same developed millivolts across the shunt.

The charger will think its doing a mighty job, but in reality its putting out only a fraction of the current it thinks its putting out.

A bit of mechanical creativity might be required to accomplish this more easily, like mounting the shunt outside on the front of the charger, but its one way to get something pretty unique that may not be commercially available. If you have a range of shunts available, a few minutes with a screwdriver and spanner can shrink your monster charger down to any required size.

 

[Daddy Tanuki]

Somewhere in any battery charger there will be a current shunt to measure the current going into the battery.

A charger can be "fooled" into limiting the charging current to something less than its maximum normal full rating by just changing the current shunt.

So buy the biggest charger that is practical to use wherever the highest available shore charging power is available, then have some means of changing the current shunt to something with a lower amp rating at the same developed millivolts across the shunt.

The charger will think its doing a mighty job, but in reality its putting out only a fraction of the current it thinks its putting out.

A bit of mechanical creativity might be required to accomplish this more easily, like mounting the shunt outside on the front of the charger, but its one way to get something pretty unique that may not be commercially available. If you have a range of shunts available, a few minutes with a screwdriver and spanner can shrink your monster charger down to any required size.

a shunt measure resistance across itself correct? jsut add a variable resistor pot in their and experiment a bit.

 

[Warpspeed]

a shunt measure resistance across itself correct? jsut add a variable resistor pot in their and experiment a bit.

That will only work if you want to increase the charger output current above what it was originally, which is not a good idea.
We want to decrease the charger output current, so we need to fit a higher shunt resistance.
We need to have the same millivolt output across the new shunt, but at a lower shunt current.

 

[Daddy Tanuki]

That will only work if you want to increase the charger output current above what it was originally, which is not a good idea.
We want to decrease the charger output current, so we need to fit a higher shunt resistance.
We need to have the same millivolt output across the new shunt, but at a lower shunt current.

just proving how little I know at any given time....

 

[l00semarble]

This thread got incredibly overcomplicated and like many the OP just vaporizes after asking a question. OP never even said what batteries he has so hard to give advice on a charger.

 

[jdege]

This thread got incredibly overcomplicated and like many the OP just vaporizes after asking a question. OP never even said what batteries he has so hard to give advice on a charger.

I'm still here and still following.

But I'm not seeing much that addresses the question.